(1) Field of the Invention
The present invention relates to a site diversity receiving method for use-with a mobile communications system and to a base-station host apparatus used in a mobile communications system employing the site diversity receiving method. More particularly, the present invention relates to a site diversity receiving method for use with a mobile communications system and to a base-station host apparatus used in a mobile communications system employing the site diversity receiving method, both of which are suitable for use with a mobile communications system such as an automobile cellular phone system or a portable telephone system.
(2) Description of the Related Art
In a mobile communications system, such as an automobile cellular phone system or a portable phone system, since a terminal station usually carries out communication while in a moving state, a propagation environment between the base station and the terminal station changes constantly. For example, the propagation environment changes due to fading or the like during a short period of time, deteriorating the quality of communication between the base station and the terminal station.
A representative countermeasure against such a phenomenon is a space diversity method in which the quality of a received signal is improved by composing into one signal (or selection of one of) a plurality of signals received from one base station via several different propagation paths.
However, from the viewpoint of a comparatively long period of time, the space diversity method cannot be expected to yield the effect of improving the quality of a received signal if the terminal station enters the shadow area produced by an obstacle such as a building and it becomes difficult for the terminal station to transmit/receive a signal to or from one base station. For these reasons, the space diversity method cannot be said to be an effective countermeasure. In view of the foregoing, a site diversity method has recently been increasingly adopted in the field of mobile communications system, which method permits the terminal station to simultaneously transmit or receive a signal to or from a plurality of base stations.
More specifically, under the site diversity method, a plurality of base stations receive a signal (or data) from one terminal station, and the data received by the respective base stations are processed and transmitted to an exchange (a higher-level apparatus in comparison with the base station and hereinafter often referred to as a base-station host apparatus). In the exchange, the received plurality sets of received data are selected or composed into one signal.
FIG. 26 is a schematic representation showing the concept of the site diversity method by which communication is established between a terminal station and a plurality of base stations. In FIG. 26, reference numerals 101 and 102 designate base stations; 103 a building (or an obstacle) existing in a communications area of the base station 101; and 104 a terminal station.
For example, if the terminal station 104 enters a shadow area produced by the building 103 while it is in the course of communicating with the base station 101, the presence of the building 103 makes it difficult for the terminal station 104 to smoothly transmit or receive a signal to or from the base station 101. However, the site diversity method permits the base station 102 adjacent to the base station 101 to also receive a signal from the terminal station 104. Consequently, in place of the base station 101, the base station 102 can communicate with the terminal station 104. As a result, the terminal station 104 can realize high-quality communication without being affected by the propagation environment (such as the presence of the building 103).
Further, under the site diversity method, since a plurality of base stations receive a signal from the terminal station 104, the minimum transmission level (or power) of the terminal station 104 required by the network to maintain the quality of communication is reduced, enabling a reduction in the power consumed by the terminal station 104. Moreover, if the site diversity method is applied to a mobile communications system employing a CDMA (Code Division Multiple Access) method, there can be expected a reduction in interference power, which in turn allows the capacity of a subscriber's line to increase. In short, the application of the site diversity method to the CDMA method is very effective.
FIG. 27 is a block diagram showing one example of a mobile communications system to which the CDMA method and the site diversity method are applied. The mobile communications system 110 shown in FIG. 27 comprises a terminal station 111, base stations 112-1 to 112-n ("n" is a natural number), and an exchange 113.
In general, a wireless line of low quality (i.e., one which may be susceptible to circuit failures) is used for communication established between the terminal station 111 and the base stations 112-1 to 112-n, whereas a wired line of high quality (i.e., substantially without circuit failures) is used for communication established between the base stations 112-1 to 112-n and the exchange 113. Because of this fact, such a communications system adopts a technique called error correcting code (ECC) in order to improve the quality of the circuit between the terminal 111 and the base stations 112-1 to 112-n. The following techniques can be conceived as the error correcting code technique.
(1) Technique 1
As shown in FIG. 28, the terminal station 111 is provided with an error correcting encoder (ENC) 111a, and the base stations 112-1 to 112-n are respectively provided with error correcting decoders (DECs) 112a-1 to 112a-n. Further, the exchange 113 is provided with a selection section 123a for the purpose of making a selection from the signals decoded by the DECs 112a-1 to 112a-n of the base stations 112-1 to 112-n.
In the mobile communications system 110 shown in FIG. 28, the signal subjected to error-correction encoding processing performed by the ENC 111a of the terminal 111 is received by the respective base stations 112-1 to 112-n via the wireless line. The signals are subjected to error-correction decoding processing performed by the DECs 112a-1 to 112a-n of the base stations 112-1 to 112-n. The thus-decoded signals are transmitted to the exchange 113 via the wire circuit. In the exchange 113, a decoded signal (with minimum deterioration) is selected from the received and decoded signals as a received signal.
Although in the foregoing description any one of the decoded signals sent from the respective DECs 112a-1 to 112a-n is selected, there may be a case where the exchange 113 is provided with a composing section in place of the selection section 113a, and the decoded signals from the error correcting decoders 112a-1 to 112a-n are composed into one signal.
(2) Technique 2
As shown in FIG. 29, the terminal station 111 is provided with the ENC 111a, and the exchange 113 is provided with a composing section 125a and an error correcting decoder (DEC) 125b.
In the mobile communications system 110 shown in FIG. 29, the base stations 112-1 to 112-n receive a signal from the ENC 111a of the terminal station 111 via the wireless line, and the thus-received signals are transmitted to the exchange 113 from the base stations 112-1 to 112-n via a wire circuit without being subjected to the error-correction decoding processing. In the exchange 113, the signals (error-correction coded signals) received from the base stations 112-1 to 112-n are composed into one signal. The thus-composed signal is then subjected to error-correction decoding processing performed by the DEC 125b. Although the signals from the respective base stations 112-1 to 112-n are composed into one signal by the composing section 125a of the exchange 113, any one signal may be selected from the signals.
The base stations 112-1 to 112-n employed by technique 2 are also capable of acquiring soft determination information such as a receiving level when receiving the signal from the terminal station 111 and of sending the soft determination information to the exchange 113 together with the received signals. In such a case, the exchange 113 can merge into one signal (or select one signal from) the signals received from the base stations 112-1 to 112-n by utilization of the soft determination information, enabling improvement in the quality of the received signal.
However, according to the foregoing technique 1 (i.e., in the mobile communications system 110 shown in FIG. 28), the signal subjected to error-correction encoding processing performed by the ENC 111a of the terminal station 111 is subjected to error-correction decoding processing performed by the base stations 112-1 to 112-n. The thus-decoded signals are subjected to selection (or composed into one signal) by the exchange 113. For example, if the decoded signals include many errors, it is impossible to obtain superior-quality data even by making a selection from the decoded signals (or by composing the signals into one signal), thereby trading off the effect expected from employment of the site diversity method and the error correcting encoding method.
More specifically, the error correcting decoding method has a great effect of improving the quality of a signal having few errors. In contrast, the method is characterized by the feature that it will increase errors if the signal includes an extremely large number of errors. If all the decoded signals include many errors, the exchange 113 eventually selects a signal that includes many errors. Further, in a case where the exchange 113 composes into one signal the signals decoded by the DECs 112a-1 to 112a-n, if some of the decoded signals include many errors, the quality of the superior-quality signals is deteriorated by the signals that include many errors. Therefore, there is a strong likelihood that only those signals that include many errors will be obtained as a composed signal.
In contrast, according to technique 2 (or in the CDMA communications system 110 shown in FIG. 29), since the signal is decoded by the exchange 113 after the plurality of sets of data have been composed into one data item, the amount of signal data flowing through a wire circuit connecting the base stations 112-1 to 112-n to the exchange 113 is increased. For example, if an encoding rate used for the error correcting encoding processing is set to 1/2, the amount of signal data flowing between the base stations 112-1 to 112-n and the exchange 113 becomes twice as large as that required by technique 1.
Accordingly, in comparison with at least technique 1, technique 2 has a disadvantage of increasing the capacity of the wire circuit connecting between the base stations 112-1 to 112-n and the exchange 113 and hence adding considerably to the cost.
Further, according to technique 2, the error-correction coded signals received from the base stations 112-1 to 112-n are composed into one signal (or a signal is selected from the signals) by utilization of soft determination information, enabling an improvement in the error rate of the received signal data. In this case, the amount of signal data flowing between the base stations 112-1 to 112-n and the exchange 113 is increased.
For example, in a case where the soft determination information is added to the received signals without changing the encoding rate (i.e., with the current encoding rate of 1/2), eight bits of soft determination information are added per bit to the received signal. In this case, the amount of signal data flowing between the base stations 112-1 to 112-n and the exchange 113 is increased to eight times that required by technique 1. In any event, technique 2 is impractical from the viewpoint of cost.